Conventionally, a spark plug for ignition is used for an internal-combustion engine. A conventional spark plug is comprised of: a center electrode having an electrode for spark discharge on a front end side thereof; an insulator accommodating the center electrode in its axial bore; and a metal shell surrounding and holding the insulator in a radial direction thereof. An end portion of a ground electrode is joined to the metal shell, and a spark discharge gap is formed by the other end portion of the ground electrode and a front end portion of the center electrode. A spark is discharged in the spark discharge gap to thereby ignite an air-fuel mixture.
In a direct injection engine, an injection orifice of an injector for injecting a fuel is exposed to the combustion chamber. When the above spark plug is mounted on the direct injection engine, and when a fuel directly strikes the spark plug, short-circuit may occur in the spark discharge gap due to a droplet of fuel, which causes a so-called fuel bridge. As a result, there is a possibility to cause a misfire. In order to avoid such a phenomenon, the injection orifice is disposed so that the fuel is injected towards an inner wall or a piston of the combustion chamber. Furthermore, by using a compressed air when the piston moves upward, an injection pressure, an amount of injection and an injection timing of the injector or the like are controlled so that a rich fuel layer reaches to a perimeter of the spark discharge gap at the time of ignition. In this way, when the injected fuel strikes against the inner wall or the piston of the combustion chamber, a droplet of incompletely vaporized fuel (i.e., any fuel not vaporized when the injected fuel is atomized and mixed with air to form the air-fuel mixture) adheres thereto, and a vaporized fuel (a splay-like injected vaporized fuel contained in the air-fuel mixture) is separated. Thus, the vaporized fuel reaches to the spark discharge gap, and prevents the fuel bridge. Further, a configuration in which the ground electrode is surrounded by a wall surface (protective member) may be employed (e.g., refer to Patent Document 1) so that the droplet-like incompletely vaporized fuel is unlikely to enter in the spark discharge gap, even though it reaches to the spark discharge gap. Patent Document 1 Japanese Patent Application Laid-Open (kokai) No. 2006-228522.